The invention relates to a camera tube having an electron source and a target to be scanned on one side by an electron beam emanating from the source. The opposite side of the target is illuminated with radiation forming the image to be detected. The target comprises a signal electrode and a selenium-containing vitreous layer containing arsenic. The signal electrode is on the radiation-receiving side of the target and the vitreous layer is on the side to be scanned by electrons.
A camera tube of the kind described above is known, for example, from British Patent Specification No. 1,135,460.
In practice, glass-stabilizing additions to selenium-containing vitreous photosensitive layers, for example arsenic, are desired so as to control the deterioration of the properties of the layers as a result of crystallization phenomena.
In addition, it is of importance, inter alia, to assure a good operation of the camera tube, that there should be good blocking against the injection of holes from the signal electrode into the selenium-containing layer so as to keep the dark current and the lag low. However, the dark current and the lag may be considerable if glass-stabilizing additions are used in high concentrations.
In low concentrations of these additions, an annoyingly high operating voltage may be necessary and a moderate sensitivity, in particular to light of long wavelength may occur.
The selenium-containing layer with additions may be separated from the signal electrode by a vitreous layer of pure selenium so that good blocking is obtained between the signal electrode and the selenium-containing layer with additions. The disadvantage of this solution, however, is that as a result of the use of a layer of pure selenium the glass stability is reduced and the lag is increased.